1. Field of the Invention
The present invention relates to tape guide apparatus and, more particularly to a tape guide suitable for use in a video tape recorder (VTR) or the like.
2. Description of the Prior Art
Tape guides for use in video tape recorders or the like are roughly classified into rotary tape guides and fixed tape guides.
The rotary tape guides are advantageous in that they impose less resistance to tapes guide thereby. However, the speeds of travel of the tapes guided by the rotary tape guides tend to reflect irregularities in the rotational speeds of bearings used in the rotary tape guides. Furthermore, if the direction in which a tape travels when it is guided by a rotary tape is not perpendicular to the axis of rotation of the rotary tape guide, then the tape is subjected to a transverse force applied by the tape guide. The latter drawback is aggravated when the tape is transversely shifted until an edge thereof is damaged by the contact with a flange of the rotary tape guide, for example. Therefore, rotary tape guides are required to be machined and assembled with high accuracy, and hence cannot be manufactured easily.
The fixed tape guides allow tapes guided thereby to run stably, but present large resistance to the running tapes.
There has been a demand for a fixed tape guide which imposes smaller resistance to a running tape. One of such fixed tape guides that meet such a demand is an air tape guide for ejecting air from small holes defined in the surface of a guide body to float a tape off the guide body for thereby reducing the resistance applied to the tape. The air tape guide is still problematic since a compressor is required as an air pressure source.
To eliminate the drawbacks of the conventional tape guides, the assignee of the present application has previously proposed an ultrasonic vibration tape guide device as disclosed in Japanese Patent Application No. 2-103627. The ultrasonic vibration tape guide device employs an ultrasonic energy to reduce resistance to a running tape while allowing the tape to run stably as with fixed tape guides. The ultrasonic vibration tape guide device is adjustable in height. This previously-proposed ultrasonic vibration tape guide device will be described below with reference to FIG. 1 of the accompanying drawings.
As shown in FIG. 1, the ultrasonic vibration tape guide device, generally designated by the reference numeral 1, includes a main shaft 5 mounted vertically on a base 18, and an ultrasonic vibrator 3 fixed to a guide member 2 that is supported on support teeth 7b of a cylindrical support shaft 7. The assembled body of the guide member 2 and the ultrasonic vibrator 3 is referred to as a guide member system. Lower and upper flanges 9 and 10 are disposed in abutment against lower and upper ends, respectively, of the support shaft 7, for guiding opposite edges of a tape wound around the guide member 2.
The main shaft 5 extends through the lower and upper flanges 9, 10 and the support shaft 7. A height adjustment screw 6 is fitted in an inner surface of the upper end of the support shaft 7, and threaded over a screw 23 formed on the upper end of the main shaft 5.
The upper flange 10 is fastened to an upper end surface of an attachment 8 by a screw 15. The lower flange 9 is fixed to a lower end surface of the attachment 8 by fixing pins 22, 24.
The attachment 8 has an ultrasonic vibrator storage space 8a defined therein which houses the ultrasonic vibrator 3 therein. As shown in FIG, 2 of the accompanying drawings, the ultrasonic vibrator storage space 8a is defined as a hole in the shape of a rectangular parallelpiped between side walls 8b having respective stopper insertion holes 8c defined therein.
Disc-shaped stoppers 39 made of rubber have engaging protrusions 39a fitted respectively in the stopper insertion holes 8c. The ultrasonic vibrator 3 is sandwiched between the stoppers 39 to prevent the guide member 2 from rotating.
The attachment 8 keeps the lower and upper flanges 9, 10 parallel to each other and spaced from each other by a distance that is about 0.1 mm larger than the length of the guide member 2.
As shown in FIG. 1, the lower flange 9 is normally urged upwardly under the bias of a coil spring 35 disposed around the main shaft 5 between the lower flange 9 and the base 18. The base 18 has a pin insertion hole 20 in which there is inserted an end of the fixing pin 22 that projects downwardly from the lower surface of the lower flange 9.
When the height adjustment screw 6 is turned, the guide member 2 is adjusted in height under or against the bias of the coil spring 35.
FIG. 3 of the accompanying drawings shows standing-wave vibrations caused of the guide member 2 when an AC voltage having a frequency corresponding to a resonant frequency of the guide member system is applied to the ultrasonic vibrator 3, the standing-wave vibrations being cut along lines X--X, developed and then illustrated. Dotted lines M--M represents nodes of the standing-wave vibrations on the guide member 2 where the vibrations have zero amplitude. The nodes N on the guide member 2 are axially spaced from the ends of the guide member 2 by the distance n, and the support teeth 7b are also axially spaced from the ends of the guide member 2 by the distance n, i.e., are positioned at the nodes N.
A video tape recorder, which is one of video apparatus employing the tape guide device 1 shown in FIG. 1, is required to become smaller in size recently, and such a demand needs a tape of small size naturally. On this background, the tape guide device 1 is required to be reduced in size. However, if this tape guide device 1 is reduced in size without modification, then various problems arise in the tape guide member as:
The tape guide device 1 shown in FIG. 1 must keep the upper and lower flanges 10 and 9 parallel to each other and spaced from each other by the distance of about 0.1 mm, requiring the attachment 8 of high accuracy and many assembly parts such as screws, pins or the like. As a consequence, a tape guide device becomes large in size and hence cannot be reduced in size as expected.
Further, the upper and lower flanges 10 and 9, which are used to define the transverse direction of the tape, are normally utilized under the condition such that either of the upper and lower flanges 10 and 9 of the tape guide device 1 is brought in contact with the tape. Of the upper and lower flanges 10 and 9, either flange in contact with the tape (upper or lower flange 10 or 9) is made of a hard wear-proof material because it is requested to guide the tape for a long period of time. Such hard wear-proof material is generally difficult to be machined. Further, if the flange made of such material becomes complex in shape, it becomes expensive. For this reason, the previously-proposed tape guide device cannot be reduced in size as described above.
Furthermore, in the tape guide device 1 shown in FIG. 1, the shape of the upper flange 10 for guiding the tape becomes complex as shown in FIG. 2 and is not suitable for making the tape guide device 1 compact in size.
Problems of the guide member vibrating member, i.e., the ultrasonic vibrator 3 are as follows:
The surface of the ultrasonic vibrator 3 on which it is mounted on the guide member 2 is a curved surface of one end portion of a piezoelectric ceramic element defined in correspondence with the diameter of the guide member 2. This curved surface is bonded to the outer circumferential surface of the guide member 2 as shown by a dotted line in FIG. 1.
However, if the curved surface is formed on one end portion of the piezoelectric ceramic element, then positive and negative electrodes are exposed and hence sometimes short-circuited. Further, if the guide member is made of a conductive material, then a short-circuit occurs between the positive and negative electrodes similarly as described above.
Furthermore, since the piezoelectric ceramic element is formed of laminated layers of the piezoelectric ceramic plate and positive and negative electrodes as described above, their surfaces associated with the outer circumferential surface of the guide member 2 cannot be formed with high accuracy and also the piezoelectric ceramic element bonded to the guide member 2 tends to be peeled off. For this reason, the conventional tape guide device is not suitable for the small video tape recorder.
The ultrasonic vibrator 3 has further problems as follows:
A length of the ultrasonic vibrator 3 in the radius direction of the guide member 2, which is in contact with or bonded to and generates standing-waves in the guide member 2 considerably influences the increase or decrease of friction coefficient .mu. of friction generated by the contact of the tape with the guide member 2. Accordingly, the length of the ultrasonic vibrator 3 extended along the diametrical direction of the guide member 2 is selected to be enough to minimize the friction coefficient .mu. of friction generated by the contact between the tape and the guide member 2.
However, in the tape guide device 1 shown in FIG. 1, the length (shown by reference letter L in FIG. 1) of the ultrasonic vibrator 3 extended along the diametrical direction of the guide member 2 is not selected to be optimum, resulting in efficiency at which the friction generated by the contact between the tape and the guide member 2 is reduced being deteriorated. There is then the disadvantage such that a temperature on the ultrasonic vibrator 3 is increased considerably.